With good electrical insulating properties, heat resistance, and mechanical characteristics, polyimides have been used in a variety of fields including semiconductor production. Polyimides generally tend to be insoluble in solvents and thermally infusible and accordingly, difficult to mold or process directly. For film formation, therefore, a solution (hereinafter, referred to as varnish) containing polyamic acid as a precursor of a polyimide is commonly used to form polyimide film through coating and curing steps for conversion. Such a varnish may be a solution as obtained from polymerization of polyamic acid or may be prepared by dissolving polyamic acid in a solvent.
In general, mechanical characteristics (elongation percentage and ultimate stress) of polyimide film can be improved effectively by increasing the degree of polymerization of the polyimide. As the degree of polymerization of polyamic acid increases, however, the viscosity of the polymerization solution increases, often causing troubles in the polymerization process. In addition, it will be difficult to adjust the varnish to a viscosity suitable for coating. The viscosity of a varnish can be adjusted appropriately through control of the polymerization degree of polyamic acid by changing the molar ratio between the acid anhydride group in the acid dianhydride monomer used and the amino group in the multivalent amine compound or diamine compound during the polymerization of polyamic acid. However, polyimides produced from this varnish have the same polymerization degree as the polymerization degree of the original polyamic acid, making it impossible to achieve high mechanical characteristics.
In this regard, Patent documents 1 and 2 have disclosed methods in which diamine is added to a varnish of polyamic acid having an acid anhydride group at a chain end so that the molar ratio between the acid anhydride group in the dianhydride and the amino group in the multivalent amine compound or diamine compound is adjusted to one.
In the methods described in Patent documents 1 and 2, the degree of polymerization of polyamic acid is controlled by changing the molar ratio between the acid dianhydride and the diamine compound to allow the viscosity of the varnish to be adjusted appropriately. In addition, the diamine added reacts with the polyamic acid during curing of coat film, thereby providing a polyimide with a high degree of polymerization.